Aromatic compounds magnetic properties

Aromaticity remains a concept of central importance in chemistry. It is very useful to rationalize important aspects of many chemical compounds such as the structure, stability, spectroscopy, magnetic properties, and last but not the least, their chemical reactivity. In this chapter, we have discussed just a few examples in which the presence of chemical structures (reactants, intermediates, and products) and TSs with aromatic or antiaromatic properties along the reaction coordinate have a profound effect on the reaction. It is clear that many more exciting insights in this area, especially from the newly developed aromatic inorganic clusters, can be expected in the near future from both experimental and theoretical investigations. 

Aromaticity, defined as a structural feature, was used as a predictive tool for compounds that had not been prepared previously, whereas reactivity, bond length, or magnetic criteria had to await the isolation of a compound and its experimental investigation. Only recently has the development of quantum-chemical methods reached the point where one can predict with sufficient accuracy the magnetic properties, the bond lengths, and the reactivity patterns of aromatics. The multidimensional character and the definition and measurement of aromaticity generated confusion and conflicts.43 A recent review discussed the multidimensional character of aromaticity and theoretical and experimental approaches to aromatic structures and their predictions, and references are indicated extensively.66... 

Compound ASE or RE" Bond lengths (A) Magnetic properties Degree of aromaticity Ref. 

The aromaticity of the different tautomers of compound 263 has been studied comprehensively by Garcia de la Vega at the density functional theory level, using double-zeta double-polarized basis sets [215], Using geometry-based index HOMA and magnetic properties-based NICS, the aromaticity of the whole rings and the different contours was evaluated. 

Aromatic compounds display magnetic properties related to a ring current. These include unusual chemical shifts, especially of protons, large magnetic anisotropies, and large diamagnetic susceptibility exaltations. 

Among the many classes of chiral molecules, helical systems are particularly fascinating. Their structure is relevant to proposed mechanisms of handedness induction in relation to chiral amplification [76], Helicenes ([A]-H) are helical molecules formed from A-ortho-fused benzene rings (Fig. 8) which display considerable rotatory power [77]. Helicenes are presently the subject of intense synthesis efforts that try to functionalize these molecules in order to attain enhanced electric, magnetic, and optical properties [78, 79]. Phenylenes ([A]-P), or heliphenes, constitute another class of helical aromatic compounds for which syntheses have recently been reported [80, 81]. They are made up of N benzene rings fused together with N - 1 cyclobutadiene rings (Fig. 8). 

The technique of principal component analysis was applied to a set of 12 characteristics associated with aromaticity, including the two above, using a variety of heterocyclic compounds <89JA7>. Three principal components, were found that accounted for most of the variance of the data. Characteristics associated with classical aromaticity, such as the two mentioned above, were dominated by the first component and those associated with magnetic properties, such as the molar susceptibility, were dominated by the second. The In score for oxazole was low, of 16 compounds, only furan was lower. On the other hand, the ii2 score was one of the higher scores. 

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